Gas chromatography is a well-known method for identifying the chemical composition of a material sample and has found application in a variety of industries which rely on the identification of chemical compounds. The gas chromatography process involves vaporizing and introducing a material sample into a chromatographic column, wherein the material sample is transported through the column by the flow of an inert, gaseous carrier, such as nitrogen (N2), hydrogen (H2) or Helium (He).
Modern gas chromatographs typically utilize fused silica capillary columns to effect separation of the sample material. When using such columns, it is often necessary to split much of the sample in order to avoid detector saturation or phase saturation of the column, or to split much of the supplied gas in order to avoid elevated baselines caused by residual solvent vapor or low vapor pressure matrices e.g. oils. In effect, the majority of consumed gas is not directly involved with the chromatography, and is exhausted to the atmosphere. For instance, a typical gas chromatograph splits 50 ml/min or more of gas and utilizes, perhaps only 1 ml/min chromatographically, corresponding to a split ratio of 50:1. Accordingly, such a chromatograph will vent 50 times the amount of gas actually required to carry a sample through a chromatograph column for an analysis. An automated feature found on many gas chromatographs typically referred to as “gas saver” allows setting lower split flows following an injection in order to conserve gas. While the gas saver feature results in consuming lower amounts of helium, it is known that there is a tradeoff between using low split flows and the subsequent contamination level (e.g. elevation of baselines) which ensue. It is therefore analytically desirable to maintain higher split flows, yet economically desirable to use lower ones. It is therefore desirable to use high split flows of 50 standard cubic centimeters per minute (sccm) or greater, and provide a means for recycling the majority of spent gas.
There have been some descriptions of systems that employ carrier gas recycling. For example, U.S. Pat. No. 4,230,464, issued in the names of inventors Bonmati et al., describes an industrial scale preparative chromatograph using high gas flow rates and large quantities of carrier gas (between 5 and 200 cubic meters of carrier gas per hour). The purification applies to the gas which has been used for the chromatographic process of a large scale physical separation and purification of known constituents (as opposed to a laboratory analysis setting, which is directed towards identifying and quantifying trace chemical constituents in widely varying matrices). U.S. Pat. No. 6,063,166, issued in the name of inventor Wilson, describes closed loop recirculation of hydrogen gas in a system utilizing metal hydride storage systems. United States Patent Application Publication 2007/0125233 A1, in the names of inventors Bostrom et al., describes field portable “down hole” instruments for in-situ analysis of subterranean fluids, that uses fixed-temperature metal hydride reservoirs as sources and storage reservoirs of carrier gas. U.S. Pat. No. 6,074,461, issued in the name of inventor Wilson, teaches the use of gas recycling systems coupled to chromatographs, wherein the recycling systems include first and second stages for effecting respective tasks of carrier gas purification and carrier gas pressurization. Wilson further notes that the gas purification stage may be designed according to the particular carrier gas operable in the particular embodiment of the chromatograph and may include a packed trap, such as molesieve, a membrane or similar device permeable only by hydrogen, a helium getter, a packed bed trap designed for cleaning helium optimized for another carrier gas, or a polymer barrier that is efficient for transmitting only helium. The aforementioned gas purification methods of the prior art suffer from inefficiency and poor scalability in the case of Bonmati et al. and suffer from cost, complexity, analytical deficiencies and/or safety in the cases of Bostrom et al. and Wilson. Such is the case when considering a recycling system based on hydrogen.
Hydrogen, when used as a gas chromatograph carrier gas, presents a potential fire or explosion hazard and is associated with some other analytical deficiencies. It is known, for example, that, if hydrogen carrier gas is employed for gas chromatography/mass spectrometry (GCMS) applications, sensitivity is reduced and adverse chemical reactions in the inlet (e.g. hydrogenation) or the ion source (e.g. de-hydrohalogenation) can occur. Therefore, for many routine laboratory or field-based analytical purposes, it is desirable to use helium exclusively as the carrier gas. Unfortunately, the increasing cost of helium is resulting in the use of this gas as a carrier for gas chromatography to become prohibitively expensive, particularly in some developing countries where, for instance, up to 500 Euros may be spent on a single cylinder of gas. Traditional methods of gas purification described in the prior art (e.g. helium gettering) utilize reactive metal alloys for ensuring removal of trace contaminants from otherwise pure helium. This technique is impractical for scrubbing multiple microliter quantities of solvents due to the limited capacity and non-reversible chemical reactions which occur in these types of traps. Likewise, molecular sieve traps of conventional design are ubiquitously employed and useful for removal of trace contaminants which are strongly adsorbed, but lighter, more-weakly-bound chemicals can break through the traps in relatively short time intervals unless large quantities of adsorbents are utilized, or cryogenic conditions are maintained around the trap. Due to the high cost of synthetic porous polymers, large-capacity in-line traps of this nature are therefore also impractical.
The compression cycle involved for recompression of the recycled gas stream is necessarily a closed loop system so as to prevent the introduction of atmospheric gasses which would otherwise contaminate the gas stream. Prior art pumping systems which employ dedicated rotary vane pumps, piston pumps etc. have the disadvantage of cost and the propensity to introduce hydrocarbon contaminants into the gas stream due to the need for oil based lubricants. Additionally, these pumps are free running type pumps which impose a vacuum on the gas harvesting side of the pump. Without special modifications to the electronic pressure control of the gas chromatograph to deal with the reduced pressure, or without providing a complex means of throttling the flow delivered to the pump, these methods are unusable for existing installations of GC and GCMS units.
Thus, it is further desirable to employ a helium reclamation and recycling system in most existing routine laboratory or field-based chromoatographs. To increase portability and versatility and reduce operating costs as much as possible, the helium reclamation and recycling system should (a) be readily adaptable to virtually any analytical gas chromatograph system without interference in the normal operation of the chromatograph system (b) should include provisions for periodical self analysis of the purity of the reclaimed helium and (c) should allow re-generation of the cleansing qualities of the reclamation system so that large quantities of sorbents are not needed. The present invention addresses such needs.